1. Field of the Invention
The present invention relates to weldable paint-coated steel sheets applicable to electric resistance welding and having excellent corrosion resistance.
In recent years much salt is used for preventing icing of snow on highway roads, and corrosion problems due to the salt have been more and more serious in automobile industries, and for overcoming these problems automobile makers have been making their all efforts in improvement of the corrosion resistance of automobiles by adopting chemical conversion treatments, improving the paint coatings themselves and structural designs of automobiles as well as by adopting surface-treated steel sheets. As for the surface treated steel sheets used for rust prevention in automobiles, metal coated steel sheets, such as zinc or zinc alloy coated steel sheets, as well as weldable paint-coated steel sheets coated with zinc-rich paints are widely used.
In the case of the zinc-rich paint coatings, adhesion after working, weldability and corrosion resistance depend on the contents of zinc powder contained in the paints. Thus, from the point of weldability, a larger content of zinc powder is more desirable, but lowers the adhesion and if the zinc powder content exceeds about 80 weight percent, the paint coatings are very likely to peel off or break into powder, a notorious phenomenon widely known as "powdering", particularly when paint-coated steel sheets are formed into doors, fenders and so on, thus hindering the press forming operation.
On the other hand, regarding the corrosion resistance, it is well known that the so-called sacrifying corrosion resistance cannot be effected unless the zinc powder is contained in amounts more than 90 weight percent, but such large zinc powder contents practically prohibit the press forming operation.
As briefly explained above, it is quite difficult to satisfy all of the adhesion, the weldability and the corrosion resistance with weldable paint-coated steel sheets coated with zinc powder containing paints.
For these reasons, the general tendencies have been that paint-coated steel sheets with preference given to the adhesion after working have been commercially produced.
Therefore, paint-coated steel sheets now commercially available are inferior in the rust preventiveness and where the base metal is exposed due to damage of the paint coatings or cutting of the sheets, red rust can quickly be formed in a short time.
2. Description of Prior Arts
In order to overcome the above defects and difficulties, the present inventors proposed paint-coated steel sheets produced by applying a paint containing a small amount of zinc powder on zinc metal coated steel sheets, as disclosed in Japanese Patent Publication No. Sho 54-11331, and these paint-coated steel sheets have been found satisfactory to overcome the above defects and difficulties.
However, further studies on adequateness of the properties of these paint-coated steel sheets for use in automobiles have revealed that when these sheets are used at parts, such as hem worked parts of doors, which are subjected to deposition of water or dust all the time, the base zinc coating is selectively attacked and corrosion products accumulate beneath the paint coating to creep back the paint coating, thus finally causing peel off of the paint coating and nullifying the rust preventive force.
The above phenomenon can be explained from the high dissolution rate of zinc in a solution containing salts. For example, it is known that in a salt spray test specified by JIS-Z-2731, the zinc coating dissolves at an abnormally rapid rate as high as about 1 g/m.sup.2. hr and this abnormally rapid dissolution rate of the zinc coating is attributed for the creep back and peel off of the paint coating, and hence the final loss of the rust preventiveness.
The present inventors have made extensive studies in order to overcome the above defect of the weldable paint-coated steel sheets using zinc coated substrates, and found that if the zinc coated substrate is replaced by a zinc alloy coated substrate for the weldable paint-coated steel sheet, the resultant corrosion resistance can be surprisingly and incomparably improved over the corrosion resistance obtained by the paint-coated steel sheet using the zinc coated substrate.
The present invention has been completed on the basis of the above discovery.